During the new development of engines, but also when they are downsized, besides increasing the power concentration, the availability, and prolonging service life particular attention is also paid to constantly increasing the efficiency of the engines while reducing emissions. In order to satisfy these aspects, the individual engine components are often subject to greater demands than before with regard to durability and wear resistance.
An example of this are the inlet and outlet valve elements in the region of the engine combustion chamber, i.e. the valve and the associated valve seat ring, which together form a tribological system. They seal the combustion chamber and control the exchange of gases in the engine. The surfaces in this system that interact with and influence each other are exposed to extremely complex stresses caused by a cumulative load that prevails in a combustion engine consisting of mechanical, thermal, tribological and chemical stress.
At the same time, each partner in the tribological system described above must also fulfill some conditions that apply only to itself.
Thus, the valve seat ring must have high strength, in particular high resistance to deformation at moderately high temperatures (creep resistance), and high hot hardness, particularly since the outlet valves strike the valve seat more than 70 times per second. To ensure fast heat dissipation in the cylinder head and guarantee that the valve temperature is lowered, valve seat rings must also have good thermal conductivity. Last but not least, good lubricity and wear resistance are also imperative requirements for valve seat rings.
Valve seat rings with the above properties are usually created by sintering a material that is designed for sintering. The powder composition (Table 2) typically consists of a combination of a high-speed steel powder (such as the commercially widespread K3 or K1 powders) and one or more hard phases with Fe-base, optionally also Co-base, and other constituents such as solid lubricants, for instance sulfides, e.g., MoS2 or K13, and/or graphite and/or copper and/or CaF2. Such valve seat rings are often infiltrated with copper as well, to achieve a higher thermal conductivity and make them more easily workable. A disadvantage of these valve seat ring materials is that they are often quite aggressive towards the counterpart element and so cause increased wear on the valve.
The valves, and in particular the valve discs, must have good heat resistance since they are exposed to temperatures of up to 1,000° C., and good wear resistance. For this purpose, it is common to plate, harden and/or nitride the valves, particularly the valve discs, to improve the tribological properties of the system. There are also tribological systems in which the valve discs have not undergone any surface treatment.
Document U.S. Pat. No. 6,318,327B1 describes a tribological system consisting of a valve seat ring and a valve. The valve seat ring is made from an iron-based sintered material and fine inclusions of 10 to 50 wt % of a CoMoCr-based intermetallic hard phase, T 800 and T 400 for example. Solid lubricants (sulfides, nitrides, fluorides, graphite) are added; infiltration and impregnation with Cu is also described. Sintering takes place in a vacuum. This is very disadvantageous for a continuous sintering process of large quantities.
An austenitic steel (SUH35 (JIS G 431 1: 21% Cr-4% Ni-9% Mn 0.4% N-0.5% C—Fe (the rest)), which is nitrided or plated with stellite F, 6 or 12 or with K8, K10, to enhance wear resistance and thereby improve the tribological properties of the system.
The problem is that optimal properties are not reached for specific tribological systems, particularly since other valve materials are not considered. This is also significant because not only is the reliability of the system determined by the interaction between the valve disc and valve seat ring, but the valve guide must also be included in this consideration. To this extent, the limitation to just one group of valve materials results in a restriction for optimizing the material pairing.
WO 2009 024 809 A1 discloses a material for a valve seat ring in which an iron-based alloy with reduced levels of the carbides of Mo, W, V and Nb is used. This powder constitutes the largest part of the powder mixture for processing. In addition, it still includes the conventional additives for improved processing, sintering, and solid lubricants and hard phases and copper.
Besides the individual characteristics of each valve and valve seat ring, it is important for a tribological system to preserve the mechanical, physical and/or chemical interactions of the partners as minimal as possible. This is usually ensured by external lubrication via fuels, combustion products or the engine oil. If this external lubrication is reduced significantly or omitted entirely, the tribological system, which was previously exposed to a liquid or mixed friction, is increasingly exposed to a solid friction, which results in greater overall wear.